Sky & Telescope - September 2021 - 53

FILTERS: TOM DOBBINS
color saturation. Its belts appear pale
greyish brown rather than the reddish
sepia characteristic of Jupiter's belts,
while the ringed planet's tropical and
temperate zones usually feature a warm
palette of straw yellow, butterscotch,
and saffron hues.
Saturn's polar regions typically
display cooler colors ranging from olive
green to teal, aquamarine, and azure
blue. These hues aren't limited to high
latitudes; even an entire hemisphere of
the planet can display them. In a 1984
review of Saturn observations submitted
to the British Astronomical Association
between 1943 and 1981, Richard
McKim and Keith Blaxall reported that
a hemisphere begins to lose its warm
tint as it turns away from the Sun,
taking on a cool cast after a prolonged
reduction in exposure to sunlight.
In 2017, Saturn's northern hemisphere
experienced summer solstice
when the planet's north pole and the
northern face of its rings were tilted
toward the Sun (and earthbound
observers) to their maximum extent.
During last year's apparition the slow
closing of the rings became apparent
and the planet's southern hemisphere
came into view. After being shrouded
beneath the shadow of the rings for
five terrestrial years, this portion of the
globe was once again exposed to sunlight
and displayed a striking blue color.
The noted planetary observer and
astronomy historian William Sheehan
reported a similar appearance almost
three decades (or one Saturnian year)
ago. In 1992 Sheehan - who today is
also a fellow S&T contributing editor -
observed Saturn under superb conditions
through the 42-inch Cassegrain
reflector of the Pic du Midi Observatory
in the French Pyrenees. " I was astonished
by the rich colors of the planet, "
he reported, " which are washed out in
small apertures and mediocre seeing
conditions . . . The most remarkable
aspect of the planet was the strongly
bluish tint of the entire southern hemisphere,
which I had not suspected in
smaller instruments. " In 1993 Sheehan
again found that the color of that hemisphere
through both the 40-inch Yerkes
pThe ringed planet's southern hemisphere displays a distinct blue tint in this image captured on
May 15th by David Hamilton with a Celestron C14 and ZWO ASI224MC camera.
refractor and a 15-inch reflector bore
" an uncanny resemblance " to the colors
of frigid ice giants Uranus and Neptune.
The stark difference in the colors of the
northern and southern hemispheres was
still apparent to my eye as late as 1997
through my 10-inch reflector.
In Saturn's deep troposphere, photochemical
reactions between solar
ultraviolet radiation and trace gases like
methane, ethane, and acetylene produce
hazes composed of minute droplets
of liquid hydrocarbons like benzene.
As the intensity of sunlight diminishes
during autumn and winter, the
production of these aerosols dwindles,
causing the planet's upper troposphere
to become more transparent. With sunlight
able to penetrate the troposphere
to a greater depth, Rayleigh scattering
by gas molecules imparts a blue color
just as it does in Earth's atmosphere.
Methane's selective absorption of light
from the far red end of the spectrum
amplifies this effect.
The intensity of sunlight is about 90
times weaker on Saturn than here on
Earth. The distant planet's low apparent
surface brightness makes it difficult to
discern subtle color differences using
small telescopes. But the bluish cast
of the southern hemisphere should be
obvious to a keen-eyed observer through
an 8- to 10-inch instrument. Those
with smaller apertures should try a
yellow (Wratten 12), orange (Wratten
21), or light red (Wratten 23A) filter to
selectively darken the southern hemisphere
and accentuate its contrast with
its northern counterpart.
pWith the aid of color filters, observers using
instruments of modest aperture can still
discern the bluish tint of the planet's southern
hemisphere.
¢Contributing Editor TOM DOBBINS
uses historical observations to inform his
modern ones.
sk yand tele scope .o r g * SEPTEMBER 2021 53

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